Multiple condensing lens array for concentrating energy on a plurality of electron beam sources of a photocathode

ABSTRACT

The invention is an array of condensing lense for use in image tube pattern fabricating applications to provide selective and intensive illuminations of individual electron emitting sources of a multiple electron beam photocathode source.

United States Patent Terence W. OKeeite Pittsburgh, Pa.;

Charles 11. Church, Alexandria, Va. 784,763

Dec. 18, 1968 Apr. 20, 1971 Westinghouse Electric CorporationPittsburgh, Pa.

Inventors Appl. No. Filed Patented Assignee MULTIPLE CONDENSING LENSARRAY FOR CONCENTRATING ENERGY ON A PLURALITY OF [56] References CitedUNITED STATES PATENTS 3,236,707 2/1966 Lins 250/213X 95/1 3,405,61410/1968 Lin et al OTHER REFERENCES F. C. Genovesco et al., PHASE PLATELENS FOR A MULTIPLE IMAGE LENS SYSTEM, IBM Tech. Disclosure Bulletin, V.8, N0. 12, May, 1966 H. R. Rottman et al., ZONE LENS ARRAY FORFABRICATION OF MULTIPLE-IMAGE PATTERNS, V. 9,

No. 1, Jun. 1966 Primary ExaminerRoy Lake Assistant ExaminerDavidOReilly Attorneys-F. H. Henson, C. F. Renz and M. P. Lynch ABSTRACT: Theinvention is an array of condensing lense for use in image tube patternfabricating applications to provide selective and intensiveilluminations of individual electron emitting sources of a multipleelectron beam photocathode source.

, GROUND PATENTEDAPR20'197:

GROUND FIG.I.

INVENTORS Terence W. O'Keeffe 8 Charles H. Chur h Y A flww ATTORNEYWITNESSES MULTIPLE CONDENSTNG LENS ARRAY FOR CONCENTRATIING ENERGY ON APLURAILTTY OI ELECTRON BEAM SOURCES 01F A PHOTOCATHODE GOVERNMENTCONTRACT The invention herein described was made in the course of orunder a contract with the United States Air Force.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a lens system for directing light onto a photocathode andmore particularly to an array of lenses for use in conjunction with amultiple electron beamemitting source.

2. Description of the Prior Art The use of a multiple electron beamsource in an image tube application is the subject of a copendingapplication Ser. No. 784,551 of T. W. Olteeffe filed Dec. 18, 1968,entitled, A MULTIPLE BEAM ELECTRON SOURCE FOR PATTERN GENERATION, andassigned to the assignee of this invention. The multiple electron beamsource is utilized to define an array of multiple images bysimultaneously scanning the multiple beams across the surface of atarget coated with an organic etch resist. In certain applications suchas rapid exposure of high resolution patterns, the current emitted byeach of the electron beam sources must be quite high.

An obvious method of increasing the output of a photoelectron source isby increasing the illumination of the photocathode. If the photocathodeis inefiicient and a well matched source of illumination if notavailable, a great deal of nonuseful energy must be directed onto thephotocathode. This excess energy can result in detrimental performanceof the photocathode due to heating.

For a photocathode containing only a small relative area of emittingsources as in the case of the multiple electron beam source, much usefulradiation is wasted if the photocathode is totally illuminated, i.e.,most of the useful radiation falls on areas of the photocathode which donot emit electrons.

SUMMARY The invention consist of a multiple condensing lens which, whenpositioned in front of the photocathode between a light source and thephotocathode, concentrates the radiation from the light source onto theelectron emitting areas of the photocathode.

A zone plate is a system of concentric circles of diameters increasingas the square root of the natural numbers such that the areas of thesuccessive circles are equal. The circles are treated such that thecircles are alternatively transparent and opaque. The zone plate actssomewhat like a convergent lens in the sense that an intense brightimage will be formed at a point on the side of the photocathode oppositeto the light murce. The zone plate has a primary focal length which is afunction of the wavelength and the radius of the smallest circle.

in a normal zone plate alternate zones are opaque so that half of theincoming light is immediately lost. The opaque material removes thatportion of the wavefront that would produce secondary waves at the focuspoint which would destructively interfere with the waves from the zonesadjacent to it. If the alternate zones instead of removing the lightcompletely cause a 180 phase shift, then all zones would constructivelyinterfere at the focus point. Such a zone plate is referred to as a halfwave (or phase reversal) zone plate and is more efficient since noincoming light is removed by the opaque zones, and the illumination atthe focus point is therefore significantly greater.

The invention comprises the novel application of the zone plate in amultiple condensing lens device positioned in alignment with theelectron sources of a multiple electron beam source to individuallyilluminate each electron source.

State of the art resist technique permit the fabrication of an array ofzone plates of the dimension and arrangement required to match the arrayof multiple electron beam sources described in the above identifiedcopending application of T. W. O'Keeife.

It is an object of this invention to provide selective and intenseillumination of the electron emitting sources of a multiple electronbeam photocathode and thereby provide improved image tube patternfabrication efficiency and improved pattern exposure speed.

DESCRIPTION OF THE DRAWING FIG. 11 is a cross-sectional view of anembodiment of apparatus in accordance with the present invention;

FIG. 2 is a partial detailed view of an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I there isshown an image tube 10 having a cathode plate 112 and a target plate 14that are essentially planar and parallel.

On the cathode plate 12 is a photocathode 11 comprising multipleelectron beam sources 13 defined by a mask 21 and a multiple lens array30 here shown to be in axial alignment with the array electron beamsources 13 such that the incident light from a light source 20illuminating the multiple lens array 30 results in concentratedillumination of the individual electron beam sources 13 therebyproviding electron response from the electron sources 113. The targetplate 14 contains one or more workpieces 15 that have a layer of organicetch resist material 16 thereon.

The image tube approach to resist exposure has several distinctadvantages over both optical and scanning electron beam method. As aspecific case a one to one magnetically focused image tube of about 1centimeter cathode to anode target spacing, operating at about 10 KVaccelerating potential and focus field of about 1 KO established by acoil (not shown) is capable of resolving images of less than 1 micronover a working area greater than 2 centimeters in width.

Deflection coils 18 provide means for simultaneously deflecting themultiple electron beams to draw a plurality of identical patterns on thetarget plate 14.

The concept of image tube resist exposure is described in detail in thecopending patent application Ser. No. 753,373 by T. W. O'Keeffe and R.M. Handy filed Aug. 19, 1968, "Pattern Definition in Resist Layers byApplication of an Electron Image and assigned to the assignee of thepresent invention. The fabrication of the multiple electron beam sourceis described in detail in the copending application Ser. No. 784,801, byT. W. O Keeffe and J. R. Morris filed Dec. 18, 1968, entitled, MaskPhotocathode and Method of Making Same, and assigned to the assignee ofthe present invention.

The multiple lens array 30 is more clearly illustrated in the partialview of one surface of the cathode plate 12 illustrated in FIG. 2

Utilizing state of the art resist techniques the multiple lens plate 30consisting of a plurality of individual zone plate lens elements 32 canbe etched on a suitable material such as a thin layer of metal, i.e.aluminum in the case of a normal zone plate or can be etched directly ona quartz plate to form a half wave zone plate. In the etching of thequartz half wave zone plate the alternate circles are etched todifferent depths to produce the desired constructive interference ofadjacent zones 34 and 36. The area of each circle is identical and thediameter of the circles increase as the square root of the naturalnumber. A complete discussion of zone plates is presented Concepts OfClassical Optics by .1. Strong, published by W. H. Freeman and Company,1958.

The focal length of a zone plate is dependent on both the radius of thesmallest circle and the wavelength )t/N of the light in the cathodeplate 12, where N is the index of refraction. This relationship isexpressed as f N/x, where a is the radius of the smallest circle. lf auseful wavelength of 2537A. is to be focused on the electron sources 13located on the inner surface of a inch thick quartz cathode plate 12,the index of refraction of which is 1.50, the diameter of the smallestcircle is approximately 2 mils.

The effective concentration of light energy on the respective electronemitting sources 13 by the zone plate lenses 32 increases the intensityof the electron beams generated by the sources 13 in response to radiantenergy from the light source 20. Due to the efficient utilization of thelight source light energy the intensity of the light source may be lessthan that required in less efficient systems. This reduction in lightsource intensity minimizes adverse effects of heating which occur inless efficient systems. Also since the zone plate is wavelengthdependent only the required wavelength would be focused. The remainderof the spectrum would be ineffectively focused so that intense hotspots" would not be produced at the photoemissive surface of cathodeplate 12.

The multiple electron beam photocathode as described in the copendingapplication of T. W. O'Keeffe, Ser. No. 784,551, can be comprised of anarray of elements each including a uniformly arranged plurality ofvarious size electron emitting sources. These micron size electronsources of each photocathode element are so arranged that a single zoneplate can be utilized to illuminate any of the electron sources withinan element. The selection of the source to be illuminated in eachphotocathode element can be controlled by radially displacing the zoneplate array 30 with respect to the light source thereby changing thephotocathode areas illuminated. The relative displacement of the lightsource 20 and the lens array can be accomplished by radially shiftingthe array 30 or by radially displacing the light source 20 as indicatedby the arrows. Thisradial displacement changes the angle between thelight source 20 and each zone plate identically thereby resulting inillumination of identical electron sources in each of the uniformlyarranged photocathode elements.

ln the event the photocathode consists of a complete pattern orplurality of complete patterns which are to be exposed directly on aworkpiece, a pair of zone plates can be employed to excite electronemissive alignment marks to generate high intensity electron beams forimpingement on the workpiece surface. The workpiece will be fabricatedto include a pair of electron beam induced conductivity devices whichwill respond to the alignment electron beam to indicate properregistration between the photoemissive pattern and the workpiece.

Various modification may be made within the scope of the invention.

We claim:

1. In an apparatus for generating a plurality of substantially identicalelectron beams comprising, a multiple electron beam photocathode source,said photocathode source comprising a plurality of substantiallyidentical, individual electron emitting sources, a source of radiationenergy for exciting said electron emitting sources, and a condensinglens array disposed between said radiation source, and said multipleelectron beam photocathode, said condensing lens array including aplurality of individual lenses, each of said lenses being operativelyassociated with one of said electron emitting sources to effectivelyconcentrate energy from said radiation source onto the respectiveelectron emitting sources.

2. A device as claimed in claim 1, wherein said multiple element lensarray is comprised of a plurality of zone plates.

3. A device as claimed in claim 2, wherein said zone plates are halfwave zone plates.

4. A system for exposing high resolution patterns on an electronsensitive anode comprising:

a multiple electron beam photocathode source, said photocathode sourcecomprising a plurality of substantially identical electron emittingsources a radiation source for illuminating said photocathode, saidphotocathode illumination producing electron emission from said electronemitting sources,

electron beam control means for directing said electron beams to saidanode for exposing a plurality of substantially identical patternsthereon, and

a condensing lens array disposed between said radiation source and saidmultiple electron beam photocathode, said condensing lens arrayincluding a plurality of individual lenses, each of said lensesoperatively associated with one of said electron emitting sources toeffectively concentrate illumination from said radiation source onto therespective electron emitting sources of said photocathode.

5. A system as claimed in claim 4, wherein said multiple lens array iscomprised of a plurality of zone plates.

6. A system as claimed in claim 5, wherein said zone plates are halfwave zone plates.

7. In an apparatus as claimed in claim 1 wherein each of said individualelectron emitting sources is comprised of an array of discrete electronemitting sources.

8. In an apparatus as claimed in claim 1 wherein each of said individualelectron emitting sources is comprised of a circuit pattern in the formof a single electron emitting source.

1. In an apparatus for generating a plurality of substantially identicalelectron beams comprising, a multiple electron beam photocathode source,said photocathode source comprising a plurality of substantiallyidentical, individual electron emitting sources, a soUrce of radiationenergy for exciting said electron emitting sources, and a condensinglens array disposed between said radiation source, and said multipleelectron beam photocathode, said condensing lens array including aplurality of individual lenses, each of said lenses being operativelyassociated with one of said electron emitting sources to effectivelyconcentrate energy from said radiation source onto the respectiveelectron emitting sources.
 2. A device as claimed in claim 1, whereinsaid multiple element lens array is comprised of a plurality of zoneplates.
 3. A device as claimed in claim 2, wherein said zone plates arehalf wave zone plates.
 4. A system for exposing high resolution patternson an electron sensitive anode comprising: a multiple electron beamphotocathode source, said photocathode source comprising a plurality ofsubstantially identical electron emitting sources, a radiation sourcefor illuminating said photocathode, said photocathode illuminationproducing electron emission from said electron emitting sources,electron beam control means for directing said electron beams to saidanode for exposing a plurality of substantially identical patternsthereon, and a condensing lens array disposed between said radiationsource and said multiple electron beam photocathode, said condensinglens array including a plurality of individual lenses, each of saidlenses operatively associated with one of said electron emitting sourcesto effectively concentrate illumination from said radiation source ontothe respective electron emitting sources of said photocathode.
 5. Asystem as claimed in claim 4, wherein said multiple lens array iscomprised of a plurality of zone plates.
 6. A system as claimed in claim5, wherein said zone plates are half wave zone plates.
 7. In anapparatus as claimed in claim 1 wherein each of said individual electronemitting sources is comprised of an array of discrete electron emittingsources.
 8. In an apparatus as claimed in claim 1 wherein each of saidindividual electron emitting sources is comprised of a circuit patternin the form of a single electron emitting source.